1. Field of the Invention
The invention relates to a thin film transistor (TFT) liquid crystal display (LCD), and more particularly, to a reflective TFT LCD or a transflective TFT LCD.
2. Description of the Prior Art
As technology of electronic products develops, human-machine interfaces such as displays are demanded for higher quality. Thin film transistor (TFT) Liquid crystal displays (LCDs) are characterized by portability, low power consumption and lack of radiation pollution, and thus are popularly used in various portable information products such as notebooks, personal digital assistants (PDAs), digital cameras, mobile phones et cetera. LCDs require external light sources to display images. Depending on the type of external source, LCDs can be divided into reflective LCDs, which use ambient lights, transmissive LCDs, which use backlight modules, and transflective LCDs, which use both ambient lights and backlight modules.
In general, since the light source of a reflective LCD is ambient light, the efficiency of ambient light reflection is one of the key factors affecting the display image quality of the reflective LCD. Similarly, when a transflective LCD is operated under a reflective mode, the reflectivity of the transflective LCD also determines whether the display image is good or not. Taking the reflective LCD as an example, pixel electrodes are used as reflective surfaces in the prior art, and pluralities of bumps are formed below the pixel electrodes to give the pixel electrodes rough surfaces in order to have a preferable scattering condition.
Please refer to FIG. 1. FIG. 1 is a sectional view of a portion of a reflective LCD 40 according to the prior art, wherein only a subpixel area is shown in FIG. 1. According to a method for fabricating a reflective LCD 40 of the prior art, a first conductive layer is deposited on a glass substrate 10, and a first photolithography and etching process (PEP) is performed to form a gate 12 in each subpixel area on the surface of the glass substrate 10. An insulating layer 14 is sequentially deposited on the glass substrate 10. Then, a semiconductor layer is formed on the insulating layer 14, and a second PEP is performed to form the TFT semiconductor channels 16. A second conductive layer is then formed on the glass substrate 10 and a third PEP is performed to form sources 18 and drains 20 in order to complete the fabrication of TFTs 22.
Then, a plurality of bumps 24 are formed in each of the subpixel areas by depositing a photoresist layer over all the subpixel areas and performing a fourth photolithography, developing, and baking process. Next, a passivation layer 26 is formed on the glass substrate 10 to cover the TFTs 22 and the bumps 24. A fifth photolithography and etching process is performed to expose a portion of each drain 20. Finally, a pixel electrode 28 is formed on the bumps 24 in each subpixel, through a sixth photolithography and etching process, which is electrically connected to the exposed drain 20. Since the metal pixel electrodes 28 are used as the reflective layer of the reflective LCD 40, the pixel electrodes 28 covering the bumps 24 also have comparatively rough surfaces, which can improve the scattering efficiency to promote the display image of the reflective LCD 40. However, in the above-mentioned prior art, at least one extra photomask is needed for defining the patterns of the bumps, and then a photolithography process, a development process, and a baking process also have to be performed to form the bumps for improving the scattering efficiency. Therefore, the complexity and cost of manufacture is high.
Technology to improve the scattering efficiency of reflective LCDs can also be applied to a transflective LCD. For example, pluralities of bumps can be formed by organic or silicon nitride non-conductive materials in each subpixel after forming TFTs and transparent pixel electrodes, and a reflective layer can be further formed on the bumps in each subpixel for reflecting and scattering light. According to this method, two depositing processes of thin films, two photomasks, and two photolithography and etching processs are additionally required for fabricating the bumps and reflection layer. Accordingly, this method costs much more than that of transflective LCDs without bumps, which have the disadvantage of lower light scattering efficiency.
Thus, how to fabricate a reflective LCD or a transflective LCD with improved light reflection efficiency but with lower manufacturing cost remains an important issue for manufacturers.